Conventionally, with regard to cellulose fibers having flame resistance and the methods for manufacturing the same, cellulose fibers using a general halide-based flame retardant or phosphorous flame retardant (red phosphorus, reactive phosphorus compound or the like) have been known. Additionally suggested are the impregnation of cellulose fibers with an organic solvent that swells the cellulose fibers and an inorganic compound that is dissolved in that organic solvent, followed by drying (see Patent document 1), and cellulose fibers containing silicon dioxide (see Patent document 2). There also have been a suggestion of treating with an aluminum compound cellulose fibers obtained from viscose mixed with sodium silicate, thus forming aluminum silicate in the cellulose fibers (see Patent document 3), and a suggestion of allowing a compound having a phosphoryl group (a phosphorus compound such as a condensed phosphoric acid) to be contained uniformly in cellulose, thus forming a hydrated compound layer having aluminum as an essential component on the surface of the cellulose fibers (see Patent document 4).    Patent document 1: JP 5 (1993)-31705 A    Patent document 2: GB 1,064,271 B    Patent document 3: JP 3179104 B    Patent document 4: JP 2001-329461 A
However, the cellulose fibers disclosed in Patent document 1 are obtained by impregnating the cellulose with the inorganic compound serving as a flame retardant, and the use of the organic solvent for impregnation causes a problem of putting a load on the environment. The cellulose fibers disclosed in Patent document 2 have poor launderability because the silicon dioxide is susceptible to alkalis and eluted by an alkaline component contained in a detergent. The cellulose fibers disclosed in Patent documents 3 and 4 use the aluminum compounds. It has been suggested that aluminum may exhibit neurotoxicity and thus may pose a question of safety. In particular, the cellulose fibers disclosed in Patent document 3 are treated with a water-soluble aluminum compound, so that aluminum ions are contained in a drainage water. The aluminum ions are highly toxic to animals and plants, and thus the influence on the environment becomes a problem.
Furthermore, aramid fibers, etc. have been known as a highly flameproof material. However, there are problems in that the aramid fibers cannot be burned at the time of disposal and are not decomposed when buried in the soil.